Observed IRIS spectra (black) of Si IV (a–c) and Mg II (d–f) at three different times as shown in previous figures (horizontal dashed blue lines in Fig. 7). Uncertainties of the observed spectra are denoted by thin gray error bars. The fit to the Si IV spectra is shown in orange, with individual components indicated by colored lines (blue and red for blueshifted and redshifted components of the Si IV line, violet for O IV line). Reference spectra for Mg II are shown in green, and are taken in the non-flaring chromosphere to the north of the erupting filament. Vertical pink lines denote rest wavelengths for individual spectral lines. At 00:34:11 UT there are several components in both lines, in particular one corresponding to 250 km s⁻¹ (pink arrow in panel f and the “?” sign in panel c). See text for details.